The Astrophysical Journal, 613:L000–L000, 2004 September 20 䉷 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE AB DORADUS MOVING GROUP B. Zuckerman1 and Inseok Song1,2 Department of Physics and Astronomy, University of California at Los Angeles, Box 951547, Knudsen Hall, Los Angeles, CA 90095;
[email protected],
[email protected]
and M. S. Bessell Research School of Astronomy and Astrophysics Institute of Advanced Studies, Australian National University, Cotter Road, Weston Creek, Canberra, ACT 2611, Australia;
[email protected] Received 2004 June 30; accepted 2004 August 12; published 2004 August 23
ABSTRACT From radio to X-ray wavelengths, AB Doradus has been an intensively studied star. We have identified ∼30 nearby star systems, each with one or more characteristics of youth, that are moving through space together with AB Dor. This diverse set of ∼50 million year old star systems is the comoving, youthful group closest to Earth. The group’s nucleus is a clustering of a dozen stars ∼20 pc from Earth that includes AB Dor itself. The AB Dor moving group joins the previously known and somewhat younger and more distant Tucana/Horologium and TW Hydrae associations and the b Pictoris moving group as excellent laboratories for investigations of forming planetary systems. Subject headings: open clusters and associations: individual (AB Doradus moving group) — stars: kinematics — stars: pre–main-sequence 1. INTRODUCTION
2. OBSERVATIONS
In 2001 we began a spectroscopic search for young stars near the Sun using the double-beam grating and echelle spectrographs on the two Nasmyth foci of the Australian National University’s 2.3 m telescope (see, e.g., Zuckerman et al. 2001 for a few early results) and the Hamilton echelle spectrograph at the coude´ focus of the 3 m telescope at Lick Observatory. Some spectra have also been obtained with the High Resolution Echelle Spectrometer (Vogt et al. 1994) at the 10 m Keck telescope and with an echelle spectrometer on the 2.5 m telescope at Las Campanas Observatory. The primary goal of these observations is the measurement of radial velocity and v sin i along with the equivalent widths of the Ha and Li l6708 lines. Radial velocity, in conjunction with proper motion and parallax, enables us to calculate the three-dimensional Galactic space motions U, V, W that are essential for the identification of individual moving groups. Details of our observations and results can and will be found in Song et al. (2003) and I. Song et al. (2004, in preparation). The spectral types in parentheses listed in the fifth column of Table 1 are based on V ⫺ K colors in preference to SIMBADlisted spectral types. Most of these stars have been little investigated previously (few references in SIMBAD), and catalog spectral types are not necessarily reliable. Some, for example, HIP 26369 and 31878, are members of binary systems where a companion with consistent spectral type and V ⫺ K can be used as a fiducial point. In addition, we can classify some of the stars based on our own grating spectrometer data (I. Song et al. 2004, in preparation). Entries for radial velocity, lithium, Ha, and v sin i in Table 1 are our own measurements. Additional measurements of some of these quantities may be found in the papers listed in the references/notes column, i.e., the last column of Table 1. A question mark in this column indicates questionable membership; for example, one component of U, V, W may differ by a few kilometers per second from the group mean, or the lithium equivalent width or X-ray luminosity may be somewhat low for a typical star of age 50 Myr. We have not observed PW And or LO Peg.
The past few years have seen the identification of a variety of comoving stellar groups near the Sun with ages in the range 8–30 Myr, which is characteristic of the period during which the planets of the solar system were forming (see Jacobsen 2003, Kasting & Catling 2003, and references therein). The first such group to be identified was the TW Hydrae association, ∼55 pc from Earth. Then the somewhat nearer Tucana/Horologium association was discovered, followed soon after by the even closer b Pictoris moving group whose stars are typically ∼35 pc from Earth. With but a few exceptions, all stars in these groups and the somewhat more distant (97 pc) h Chamaeleontis cluster are located deep in the southern hemisphere (see Zuckerman & Song 2004 for a recent review). Given that nearby main-sequence stars are brighter and thus more prominent and easier to study than more distant ones, we anticipated that no substantial stellar association closer than the b Pictoris group would ever be identified. But we were wrong. We have identified the ultrarapid rotator, quintessential active binary star, AB Doradus, only 15 pc from Earth, as the most famous member of a young, comoving, stellar group that partially surrounds the Sun (Table 1). Many AB Dor group stars are found in the northern hemisphere. Current adaptive optics (AO) systems on large telescopes and the Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope (HST) become sensitive to faint objects near bright ones at separations larger than about 1⬙. As may be seen in Table 1, about half the stars in the AB Dor moving group are sufficiently close to Earth that planets within 30 AU of these stars are accessible to AO and HST. Such systems can be probed for warm (self-luminous) planets of a few Jupiter masses having semimajor axes comparable to those of the giant planets of the solar system. 1 UCLA, Center for Astrobiology/Institute of Geophysics and Planetary Physics, 3845 Slichter Hall, Los Angeles, CA 90095-1567. 2 AURA/Gemini Observatory, 670 North A’ohoku Place, Hilo, HI 96720.
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TABLE 1 The AB Dor Moving Group HIP
31711 31878 36349 63742 76768 81084 82688 86346 106231 110526 113579 113597 114066 114530 115162 118008
PW And HD 4277 HD 6569
R.A. (J2000)
00 00 01 01 HD 13482 02 02 HD 16760 02 HD 17332 02 03 03 V577 Per 03 03 03 GJ 159 04 HD 25953 04 HD 35650 05 AB Dor 05 05 UY Pic 05 HD 45270 06 GSC 8894-426 06 HR 2468 06 06 V372 Pup 07 HD 113449 13 HD 139751 15 16 HD 152555 16 HD 160934 17 LO Peg 21 GJ 856A 22 HD 217343 23 HD 217379 23 23 HD 218860 23 23 HD 224228 23
18 45 06 20 12 42 42 47 11 11 33 33 47 02 06 24 28 36 36 22 25 38 39 28 03 40 33 54 38 31 23 00 00 06 11 19 56
20.9 50.9 26.1 32.3 15.4 20.9 21.3 27.4 12.3 13.8 13.5 14.0 23.3 36.7 41.5 30.2 44.8 55.1 56.8 30.9 55.9 00.4 50.0 51.4 49.7 28.4 41.6 08.1 39.6 01.7 29.1 19.3 27.9 04.8 52.0 39.5 10.7
Decl. (J2000) ⫹30 ⫹54 ⫺14 ⫺11 ⫹23 ⫹38 ⫹38 ⫹19 ⫹22 ⫹22 ⫹46 ⫹46 ⫺01 ⫺00 ⫹01 ⫺38 ⫺65 ⫺47 ⫺47 ⫺60 ⫺60 ⫺61 ⫺61 ⫺30 ⫺05 ⫺18 ⫺09 ⫺04 ⫹61 ⫹23 ⫹32 ⫺26 ⫺26 ⫹63 ⫺45 ⫹42 ⫺39
57 58 17 28 57 37 37 22 25 24 15 15 58 16 41 58 26 57 57 13 03 32 28 14 09 41 33 20 14 20 27 09 18 55 08 15 03
Sp.
22 K2 V 40 F8 V⫹K3 47 K1 V 04 (G8) 29 K1⫹K5 22 (K3.5) 08 G5 18 G0⫹G5 24 (K6) 58 G5 26 (G5) 19 M0 20 M3 08 F5 V 03 F5 11 (K7) 55 K1 48 (K7) 53 K0 V 07 G1 V 28 M2 00 G1.5 V 41 (K7) 48 (M3) 42 (K1) 46 (K7) 12 M0.5 25 G0 16 M0 07 K8 34 M3 13 G3 V 43 (K8) 34 (M1) 11 (G5) 10 (G4) 08 K3 V
V D (mag) (pc) 8.6 7.8 9.5 8.4 7.8 10.2 8.7 6.9 10.6 8.5 8.2 11.2 11.6 5.4 7.8 9.1 6.9 9.8 7.9 6.5 11.7 6.2 9.7 10.0 7.7 10.1 11.3 7.8 10.2 9.2 11 7.5 9.8 10.9 8.8 8.9 8.2
(28) 48.5 50.0 35.1 32.3 49.6 49.6 32.6 49.8 49.8 33.8 33.8 16.3 19.2 55.3 17.7 14.9 23.9 23.9 23.5 (22) 21.7 21.9 15.6 22.1 42.6 31.9 47.6 24.0 25.1 16.0 32.1 30.0 24.9 50.5 49.4 22.1
RV (km s⫺1) … ⫺15.4 Ⳳ ⫹6.0 Ⳳ ⫹9.9 Ⳳ ⫺1.3 Ⳳ ⫺4.1 Ⳳ ⫺3.3 Ⳳ ⫹4.1 Ⳳ ⫹4.1 Ⳳ ⫹5.2 Ⳳ ⫺6.0 Ⳳ ⫺6.1 Ⳳ ⫹16.0 Ⳳ ⫹17.0 Ⳳ ⫹17.6 Ⳳ ⫹30.9 Ⳳ ⫹33.0 Ⳳ ⫹31.1 Ⳳ ⫹31.0 Ⳳ ⫹30.0 Ⳳ ⫹31.8 Ⳳ ⫹33.4 Ⳳ ⫹30.5 Ⳳ ⫹26.6 Ⳳ ⫹2.0 Ⳳ ⫺8.9 Ⳳ ⫺15.0 Ⳳ ⫺17.1 Ⳳ ⫺35.6 Ⳳ … ⫺20.6 Ⳳ ⫹6.3 Ⳳ ⫹8.4 Ⳳ ⫺23.7 Ⳳ ⫹10.3 Ⳳ ⫺19.7 Ⳳ ⫹12.1 Ⳳ
0.5 1.2 1.0 0.3 0.3 0.2 1.3 0.3 0.2 0.3 1.1 1.7 0.3 0.6 1.0 3.0 1.1 1.0 0.7 2.0 1.0 0.7 1.0 0.5 0.4 0.4 0.5 0.7 2.1 1.5 1.5 0.8 1.2 0.2 0.5
Li ˚) (mA
Ha ˚) (A
v sin i (km s⫺1)
… 119 150 145 110 146 158 155 34 145 200 30 0? 100 120 0 295 30 240 135 0 120 50 0 142 110 0 133 40 … 0 167 0 30 220 160 76
… ⫹0.91 ⫹0.72 ⫹0.76 ⫹0.80 ⫹0.23 ⫹1.09 ⫹0.90 ⫹0.10 ⫹0.87 ⫹0.70 ⫺2.50 ⫺2.20 ⫹1.08 ⫹1.20 ⫹0.28 ⫹0.10 ⫺0.80 ⫹0.54 ⫹1.05 ⫺3.90 ⫹0.84 ⫹0.26 ⫺2.35 ⫹0.74 ⫺0.55 ⫺0.60 ⫹1.18 ⫺1.09 … ⫺3.60 ⫹0.96 ⫹0.18 ⫺1.60 ⫹0.88 ⫹0.90 ⫹0.78
… 24 10 10 !10 6 27?? 13 … … 7 20 18 14 … 6 80 44 9 16 … 15 12 20 6 8 7 15 17 … 16 8 7 8 3 … 6
U, V, W (km s⫺1) ⫺4.4, ⫺8.9, ⫺8.6, ⫺4.7, ⫺7.1, ⫺8.7, ⫺9.3, ⫺8.5, ⫺5.1, ⫺6.0, ⫺6.5, ⫺6.5, ⫺7.4, ⫺7.2, ⫺6.9, ⫺7.1, ⫺7.5, ⫺7.2, ⫺7.2, ⫺7.6, ⫺10.3, ⫺7.1, ⫺7.2, ⫺7.4, ⫺5.0, ⫺7.5, ⫺7.0, ⫺6.1, ⫺5.3, ⫺5.7, ⫺6.8, ⫺3.1, ⫺2.0, ⫺6.8, ⫺7.9, ⫺4.8, ⫺7.6,
⫺27.1, ⫺26.6, ⫺31.1, ⫺27.8, ⫺28.3, ⫺28.7, ⫺28.3, ⫺28.5, ⫺28.6, ⫺28.3, ⫺25.6, ⫺25.6, ⫺27.1, ⫺28.6, ⫺27.8, ⫺27.0, ⫺29.8, ⫺27.0, ⫺26.9, ⫺26.7, ⫺27.7, ⫺28.5, ⫺27.4, ⫺24.4, ⫺28.8, ⫺31.9, ⫺28.7, ⫺28.8, ⫺27.6, ⫺27.3, ⫺27.8, ⫺26.8, ⫺24.5, ⫺27.3, ⫺29.1, ⫺27.5, ⫺27.9,
Refs./Notes ⫺16.1 1–4 ⫺15.8 3⬙. 8 binary ⫺9.3 ?; 23⬙ binary ⫺13.6 ⫺11.8 ?; 3; 1⬙. 8 binary ⫺13.1 ⫺13.4 ⫺12.9 5, 6; 3⬙. 6 binary ⫺16.1 ⫺16.7 ⫺15.7 1, 7, 8 ⫺15.7 ⫺10.6 ⫺11.6 1, 9, 10 ⫺14.3 ⫺14.5 ⫺16.0 1, 3, 8, 11; 9⬙ binary ⫺13.9 ⫺13.9 1, 3 ⫺13.6 3, 6, 12 ⫺15.6 ⫺15.0 1, 3, 12–14; 0⬙. 8 binary ⫺13.9 ⫺15.7 0⬙. 3 binary ⫺9.8 1, 3, 4, 15 ⫺15.6 ?; 0⬙. 9 binary ⫺13.4 ? ⫺12.6 3 ⫺14.5 ?; 1, 4 ⫺15.0 1, 3, 8 ⫺15.0 1; 1⬙. 8 binary ⫺14.1 ?; 3, 6 ⫺15.4 1⬙. 8 binary ⫺15.9 1 ⫺11.3 ? ⫺14.3 ⫺12.3 ?; 3
ZUCKERMAN, SONG, & BESSELL
3589 5191 6276 10272 12635 12638 13027 14807 14809 16563A 16563B 17695 18859 19183 25283 25647 26369 26373 30314
Other Name
Note.—We have not observed PW And and LO Peg; data are available in the listed references. For HIP 63742, radial velocities in the literature range from 11.5 to ⫺6.3 km s⫺1. Thus, this star may be a spectroscopic binary. If, for example, the systemic radial velocity is 2 km s⫺1, then U, V, W p (⫺6.6, ⫺26.8, ⫺12.9) km s⫺1. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Sp. p spectral type; D p distance; and RV p radial velocity. The method described in the next to last paragraph of § 3 was used to calculate the listed distances to PW And and GSC 8894-426. References.—(1) Montes et al. 2001b; (2) Montes et al. 2001a; (3) Wichmann et al. 2003; (4) Fekel 1997; (5) Nidever et al. 2002; (6) Cutispoto et al. 2002; (7) Christian & Mathioudakis 2002; (8) Jeffries 1995; (9) Chen et al. 2001; (10) Reiners & Schmidt 2003; (11) Vilhu et al. 1987; (12) Torres et al. 2000; (13) Cutispoto et al. 1999; (14) Favata et al. 1995; (15) Gaidos et al. 2000.
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TABLE 2 Nearby Young Stellar Groups Group Name
Spectral Type Range
Age (Myr)
U, V, W (km s⫺1)
TW Hydrae association . . . . . . . Tucana/Horologium . . . . . . . . . . . b Pictoris moving group . . . . . . AB Dor moving group . . . . . . . . h Cha cluster . . . . . . . . . . . . . . . . . . Cha-near . . . . . . . . . . . . . . . . . . . . . . .
A–M B–M A–M F–M B–M A–M
8 30 12 50 8 10?
⫺11, ⫺18, ⫺5 ⫺11, ⫺21, 0 ⫺11, ⫺16, ⫺9 ⫺8, ⫺27, ⫺14 ⫺12, ⫺19, ⫺10 ⫺11, ⫺16, ⫺8
Note.—See tables in Zuckerman & Song (2004) for a listing of members of these associations.
U, V, W was calculated using weighted averages of proper motions from the PPM Star and Tycho catalogs and of radial velocities measured by us and by others. 3. DISCUSSION
After combining our observations with those of other astronomers and with astrometric catalogs, especially Hipparcos, it became apparent that Table 1 stars have a similar motion through the Galaxy. The stars also have at least one indicator of youth, usually multiple indicators. These include Ha emis˚ absorption, large v sin i, large Xsion, strong lithium 6708 A ray flux, and a location above the main sequence on an MK versus V ⫺ K color-magnitude diagram (see Zuckerman & Song 2004 for extensive considerations of age diagnostics). A key to identifying youthful moving groups near the Sun is the careful measurement of U, V, W because differences among the groups are not large (Table 2). The space motion of the AB Dor group is clearly distinguished from that of the other young groups by its more negative V and W components. The existence of distinguishable proximate kinematic groups in addition to those listed in Table 2 will be discussed by I. Song et al. (2004, in preparation). We estimate the age of the AB Dor moving group in two ways. First, we compare the intensity of Ha emission (and absorption) of the late K- and early M-type stars in the Tucana association with those in the AB Dor group. For similar spectral types, the Ha lines are more strongly in emission in Tucana. Second, we plot three M-type members of the AB Dor group on an MK versus V ⫺ K diagram (Fig. 1). As can be seen, they lie slightly above the main sequence. If Tucana is ∼30 Myr old, as seems likely (e.g., Stelzer & Neuhauser 2000), then the AB Dor stars are probably ∼50 Myr old, but with at least 10 Myr uncertainty. Assuming an age of 50 Myr, one may use Figure 1, or, better, Figure 2 in Zuckerman & Song (2004), and a measured radial velocity to determine the U, V, W of stars with indications of youth but lacking accurate (Hipparcos) parallaxes. Specifically,
Fig. 1.—Mid–M-type members of the AB Dor moving group. The indicated 10 Myr and zero-age main-sequence (ZAMS) isochrones are empirically determined (see Song et al. 2003 for additional details).
one may read MK from Figure 1 for a 50 Myr old star with known V ⫺ K. Then with known mK, and accurate proper motions from, say, the Tycho catalog, one can calculate the U, V, W of the star. The U, V, W of PW And and GSC 8894-426 match those of AB Dor group members that have Hipparcosdetermined distances. The Ursa Majoris and Tucana moving groups are each characterized by a nuclear cluster of ∼10 stars and a surrounding stream of many more stars. The structure of the AB Dor group appears similar. The stars listed in Table 1 from HIP 25283 to 36349 can be regarded as the nucleus and the other Table 1 stars as stream stars. The nuclear group, which contains AB Dor itself, is deep in the southern hemisphere and at an average distance from Earth of ∼20 pc. These nuclear stars are contained in a region only ∼10 pc on a side. 4. CONCLUSIONS
We have identified a group of comoving ∼50 Myr old stars that partially surround the Sun. Spectral types range from late F-type to mid–M-type stars. A similar range of spectral types is seen in previously discovered nearby moving groups with ages in the range 8–30 Myr (Table 2; Zuckerman & Song 2004). As instrumentation improves, this ensemble of stellar associations will enable astronomers to follow the process of planetary formation over a range of stellar masses during the critical age interval from 8 to 50 Myr. This research was supported in part by NASA’s Astrobiology Institute and by NASA grants to UCLA.
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